The present invention relates to a power supply device in a motor vehicle.
Formerly, the power supply in a motor vehicle was achieved in most cases with a single battery charged by a generator. In modern motor vehicles with a plurality of electrical consumers one battery is sometimes no longer sufficient for the power supply so that two separate batteries which are connected either in series or in parallel must be used.
Since most of the electrical consumers in the motor vehicle require a supply voltage adjusted to a constant value, supply problems occur particularly during the starting process. This is because the starter has a low internal resistance and therefore overloads the vehicle supply system by a high current of more than 100 amperes so that the vehicle supply voltage drops during the starting process to a value at which e.g. the ignition or the injection no longer functions perfectly.
To avoid this problem in a vehicle supply system for a motor vehicle known from DE-OS 38 12 577, the starter is connected to a battery, while the sensitive consumers requiring constant voltage are connected to another battery. The two batteries can be supplied with voltage optionally by one or two separate generators.
However, the known vehicle supply system for a motor vehicle has the disadvantage that the connection between the two batteries cannot be interrupted when starting so that a drop in voltage at the battery connected with the starter can affect the rest of the vehicle supply system.
A further disadvantage of the known vehicle supply system consists in that the nominal voltage of the battery connected with the starter is higher than the nominal voltage of the other battery so that a discharged starter battery cannot easily be recharged by the other battery.